1. Field of the Invention
The present invention relates to a position measuring system, having a material measure or scale with a least one measuring graduation extending along a straight line or a curved line, a scanning device for scanning the measuring graduation with the aid of a light source, a detector unit of the scanning device, by which light beams transmitted from a light source and modified by the measuring graduation can be received for evaluation, and a lens arrangement, arranged between the material measure or scale and the detector unit and constituted by optical lenses, for generating a defined image of the measuring graduation on the detector unit.
2. Discussion of Related Art
Such a position measuring system is used for determining the relative positions of two objects which can be moved with respect to each other, for example two machine elements of a machine tool, which can be moved in relation to each other. For this purpose the scale is connected with one of the two objects, and the scanning device with the other one, so that by scanning the scale by the measuring graduation the amount of movement of the two objects with respect to each other along the extension direction of the measuring graduation (measuring direction) can be determined. Straight line movements of the two objects in relation to each other can be measured with so-called linear position measuring systems, wherein the measuring graduation extends along a straight line, while so-called angle measuring devices are used for determining the relative position of two objects which can be rotated with respect to each other. The measuring graduation can basically extend along any arbitrary line along which the relative movement of two objects which are movable with respect to each other is to be determined.
The different design of the measuring graduation of linear position measuring systems on the one hand, and of position measuring systems for determining a movement along a curved line, on the other, has the result that the detector unit (opto- electronic detector), by which the light beams modified by the measuring graduation (by an incident or a transmitted light method) are received for evaluation must be matched to the geometry of the respective measuring graduation. This applies in particular to the design and grid layout of the radiation-sensitive surface of the detector unit which, as a rule, is constituted by a suitable opto-electronically integrated circuit (opto-ASIC). The employment of different opto-electronic detectors for different geometries results in a corresponding increase of the manufacturing costs.
A position measuring system is known from EP 0 801 724 B1, in which an analyzer grating is arranged between the measuring graduation to be scanned by light, and the associated opto-electronic detector, in order to generate a strip pattern resulting from the light modified by the measuring graduation. In this case the analyser grating is designed in such a way that a straight-line extending strip pattern is generated by the light modified by a circle-shaped measuring graduation, or in the reverse manner a circle-shaped measuring graduation is generated from the light pattern generated by a linear measuring graduation. It is thus possible, for example in the course of scanning a circle-shaped measuring graduation, to generate a strip pattern which is to be received by an opto-electronic detector and extends straight in the same way as the strip pattern customarily generated in the course of scanning a linear measuring graduation. By this it is possible to evaluate the strip patterns from the angle measuring device, on the one hand and, on the other, those from the linear position measuring system by using opto-electronic detectors of the same type.
It is known from U.S. Pat. No. 5,486,923 to utilize a diffraction grating as a wave front-correcting structure in a position measuring system. An opto-electronic angle measuring system is known from U.S. Pat. No. 4,577,101, wherein a circle-shaped measuring graduation is reproduced on a linear opto-electronic detector by using a conical reflector and a cylindrical lens. Note that the entire contents of U.S. Pat. Nos. 5,486,923 and 4,577,101 are incorporated herein by reference.